Towhee Trouble

Alright guys, you got me. I made a mistake on my last blogpost. When I described finding some brushfinches in the mountains outside Mexico City, I said "their phylogeny is quite interesting, but maybe I'll save that topic for another blog post," and you smug bastards were on to me immediately. Against overwhelming pressure from my readers and haters, I have no choice. I admit it. I was referring to Charles Sibley's classic 1950s research on the hybrid zone between Collared and Spotted Towhees in central Mexico, and I absentmindedly thought it was a paper about brushfinches. I don't think I ever actually read the paper, I think I must have just seen it discussed somewhere. I had the image of the brushfinch-like Collared Towhee in my head and got mixed up. No one is perfect and I'm admitting my mistake, so please, take it easy with the death threats and hate mail. My inbox is overflowing!

In any event, I'd like to wipe the slate clean and dig into this research a little bit. Charles Sibley's work is blogpost-worthy for several reasons. Firstly, I get a kick out of reading these classic research papers from decades ago, and I hope you will too. Gone are the days when a biologist could go swashbuckling into Mexico with nothing but a small team of student assistants, and collect samples across the entire latitudinal gradient of the country. Sibley's paper contains a tinge of the old adventure writing style that one might find in naturalist narratives for instance from Darwin, or Bartram's Travels. 

Another amusing quirk of science writing from this time is that you can find prominent researchers with polar opposite views on now well-established subjects. Moreover, they were not afraid to express their extreme opinions with hostile confidence. In his introduction, Sibley illustrates how controversial the subject of hybridization was at the time by first quoting Edgar Anderson's view that "introgressive hybridization is certainly more important than all other factors combined in providing raw materials for natural selection to work upon," and immediately follows this with G. Ledyard Stebbins' view that "introgressive hybridization, whatever may be its importance in modifying and amplifying the variation pattern of certain individual species, is nevertheless by its very nature not a way of producing new morphological or physiological characteristics, and therefore of progressive evolution." Science is simply more fun when you have one researcher who's certain that hybridization is the most important factor by far in evolution, while another believes it isn't a factor at all. Nowadays, we know much more about this subject, and these types of arguments tend to quickly fizzle out into a middle-of-the-road "it depends." 

Getting to the meat of the paper, Sibley took on the unique and troubling case of a hybrid zone between two bird species in Mexico: the Collared and Spotted Towhee. Hybrid zones can take on many forms and result from many different situations, but one textbook process that is a useful baseline example is as follows: one freely breeding population gets bisected, possibly by some geographic change such as a river or mountain formation, and reproductive isolation ensues. Over time, the two populations diverge genetically, and eventually, through range shifts/expansion, come into "secondary contact" with each other. At this point, genetic incompatibilities have built up during their isolation, but perhaps not enough for them to consistently recognize each other as different. They interbreed, but because of these genetic differences their hybrids are inviable or less fit for any number of reasons. Total reproductive isolation is rarely achieved in nature, but if hybrids have lower fitness, various mechanisms may discourage interbreeding, and the hybrid zone will gradually grow smaller and less relevant. The exceptions to this, where stable hybrid zones are maintained instead of diminished, often end up being more nuanced and interesting. In some cases, hybrids form fairly stable and recognizable types, think for instance of Brewster's and Lawrence's Warblers, or hybrids between red-shafted and yellow-shafted Norther Flickers that end up with predictable intergrade orange shafts. What makes Collared and Spotted Towhee hybrids so unique and challenging is that they exhibit less predictable variation across several traits (Sibley identifies six key traits that vary between species), and hybrids are far-flung across a range of hundreds of miles. 

Broadly speaking, the pure-blooded Collared Towhees are in western Mexico, and moving east one begins to find specimens with more black on their cheeks and throats. Eventually, their distinctive white supercilium disappears altogether as it is filled in with black. Moving even further into the center of this range, the classic Spotted Towhee rufous flanks begin to show up. The olive green back remnant of Collared Towhees doesn't disappear in specimens until one reaches all the way over very nearly to the eastern coast (DeRaad et al. 2023, a paper I will be discussing shortly, has some terrific graphics that illustrate these various types). This is a bit of an oversimplification however, as across this range researchers found a huge amount of variability, and Sibley identifies several different clines operating at once. Despite these complications, Sibley writes that there's no doubt that this pattern of variation resulted from hybridization between two distinct species. But where did the hybrid border initially form? Why are introgressed traits so wide ranging in this case? Are the different plumage characteristics adaptive in some way? What are the implications of these results for the role of hybridization in evolution writ large? These questions Sibley offers some initial thoughts on, but for the most part he leaves them standing as a challenge to simplistic and idealized conceptualizations of the species question.  

This is a perfectly reasonable conclusion to leave us with, especially given the relatively limited tools Sibley had at his disposal. The truly astounding follow-up however, is that researchers reevaluated Sibley's hybrid zone in 2023, marshalling all the modern genetic resources available to them to answer these questions, and they still wound up short. DeRaad et al. were able to dismiss one of Sibley's hypotheses that recent human-made habitat alterations had caused the unusual hybrid distribution by showing that in the 70 years since Sibley collected his specimens, the genetic picture remained largely the same. This finding displays more stability in towhee hybrid forms than would be expected of a recently formed hybrid zone, and lends more credence to the theory that a longer time frame of hybridization caused the resulting variation. DeRaad et al. postulated that secondary contact may have occurred more than one thousand years ago. DeRaad cleans up a few of Sibley's other misapprehensions and provides a wealth of new genetic context to the picture, but ultimately finds that these towhees are pretty much just as tricky today as they were 75 years ago. 

DeRaad et al.'s results confirm that Collared and Spotted Towhees do not form a typical "tension zone," where two distinct species' ranges meet in the middle and suffer from low hybrid fitness. The hybrids are too wide ranging and have "erased nearly all fixed autosomal differences between the two ends of the cline" (DeRaad et al. 2023). They also consider the possibility that the range represents a complex of multiple semi-isolated lineages of towhees that are in the process of developing reproductive isolation, and potentially even full species status, but yet again come up short of anything conclusive. It is possible that hybrids are hybridizing with other hybrids, creating novel variation and complicating the picture even further! DeRaad painstakingly characterizes the individual trait variation and provides compelling evidence of how various traits could be adaptive in different parts of the complex patchwork of habitat, but since many traits seem to vary individually and sometimes along different gradients from other traits, it is quite difficult to explain in total. Researchers find several candidate regions where more genetic differentiation exists between populations, but never enough to confidently establish species or even subspecies delineations. 

No doubt somewhat exasperated, DeRaad et al. conclude "it is humbling to recognize that many of the novel genomic patterns we document here bear out Sibley's (1950) taxonomic recommendations." The unique biogeography of their range, and the complex and thorny patchwork of their hybrid interactions bely simple categorization, even after 75 years of study. Echoing the sentiments of a paper I covered in this blogpost, modern genetic methods, as comprehensive as they've become, do not always offer easy solutions to nature's pesky idiosyncrasies. From purely phenotypic work done 75 years ago up to the modern genomic techniques of today, the specific nature of this species relationship remain hidden away, locked in mystery, like Excalibur locked in stone. Will there ever be a hero strong enough to pull out the truth? 

Bibliography:

Sibley, Charles G. “Hybridization in the red-eyed towhees of Mexico.” Evolution (1954), vol. 8, no. 3.

DeRaad, Devon A. et al. “Hybrid zone or hybrid lineage: a genomic reevaluation of Sibley's classic species conundrum in Pipilo towhees.” Evolution; international journal of organic evolution (2022): n. pag.

Birding CDMX

The first thing to understand about Mexico City is that we Americans do not belong there. This may come as a shock to some, especially as the city is currently overwhelmed by remote-working product manager types from our great nation, but the problem relates to a spiritual rift that has been growing between our neighboring countries for centuries. To put it simply, we took their land 175 years ago and turned it into Tempe Arizona. After the Mexican-American war, we chose the path of superhighways, 3-ton pickup trucks, and salad bowl chain restaurants, which locked our national spirit into an existence that is incompatible with life in Mexico’s capital city. It is for this reason that while spending a long weekend in Mexico City two weeks ago, when I felt vaguely too tired and a little uncomfortable throughout the trip, I knew it wasn’t the cervezas, or the sudden change in altitude, or even the spicy plates of chilaquiles with which I’d start each morning. I knew deep down I was being spiritually punished for trespassing into such a wonderful place. Fleeing the concrete-mega-gas-station-manicured-lawn-sprawling-subdivision-fast-food matrix of central Florida, Mexico City’s pleasantly walkable streets lined with lush vegetation and colorfully painted buildings felt almost too good, bordering on unreal. I spent the weekend enjoying the delicious food, warm atmosphere, and of course the very good birding in the area.  

I was there with my girlfriend Nada and her family. Her parents had managed to get tickets to the World Cup match that took place the preceding Wednesday between South Korea and South Africa in Monterrey, and they very kindly invited me to meet them afterwards in Mexico City for the second half of their trip. Nada and I flew out of Orlando on Thursday night, and since I had left for a Snail Kite banding session at one of our most remote sites at 6 o’clock that morning, by the time we arrived in Mexico City at around midnight EST, I was wiped out. We had planned to rent a car for the weekend, which I had been trepidatious about, not exactly knowing the driving culture of the area. On arrival, dead-tired and wanting nothing more than to get to our Airbnb, we found that we had rented a car from the one rental agency that only had an office at the terminal across the airport from the one we landed in. Fatigue-driven irritation welling up in me, I held my nerve and found the shuttle to the relevant terminal. The shuttle bus driver informed us it would cost us to get there, and we could pay “solo en efectivo.” Of course I had not yet converted any currency, and this minor inconvenience was the final straw. Boiling over, I petulantly told Nada, “I never thought a rental car was a good idea anyway.” We decided to take a taxi to our Airbnb 20 minutes away, and the fight spiraled in the car. “You agreed it was a good idea when I planned this,” she retorted, “I did nothing of the sort,” was my reply. It got out of control from there. After getting to the Airbnb, maneuvering around Nada’s impulse to leave me and walk through the midnight streets of CDMX to her parent’s Airbnb for the night, we finally were in bed by 1 am, or 3 am EST. Needless to say we would not be getting up early for our first morning of birding in the country.  

Day One:

The beautiful weather the following morning contributed nicely to our amelioration. How could we be mad at each other as we ambled our way amongst the fragrant taco stands and mom-and-pop shops of the Roma neighborhood, as Inca Doves foraged seeds on the sidewalk, and Rufous-backed Robins sang from the trees shading our passage? Stopping at Hule café for an espresso and our first plate of chilaquiles of the trip, we finally had a chance to laugh about our Airbnb host’s obviously google-translated welcome message. Jokes about my girlfriend’s name are virtually always annoying and never welcome, but we agreed that the way our host began with “Dear Nothing,” was somewhat funny. Caffeinated and full-bellied, we embarked on the remaining 20 minutes of our walk to our first birding destination.

Bosque de Chapultepec is a large and relatively well conserved park in downtown Mexico City with an eBird list of 266 species. It is a convenient and exciting spot to bird, especially for a gringo like myself enjoying his first visit to Mexico. Entering the park, we immediately found a few more Rufous-backed Robins and a flock of Bushtits. The Bushtits there represent the unique “melanotis” or “black-eared” subspecies and caused me some initial confusion. These birds are everywhere in the city, and the first time I laid bins on them, despite immediately recognizing their “giss” (birding lingo short for “gestalt”) as “Bushtit-like,” their handsome black mask gave me pause, since the northern types that were more familiar to me are uniformly gray. I was not expecting to see such a unique subspecies, but it was a pleasant surprise.

                                                              A Rufous-backed Robin hunting for worms.

A little further into the park, Nada pointed out a seemingly orphaned group of ducklings in a small pond. Looking everywhere, I couldn’t find any adults, but by process of elimination we decided they were Mexican Ducks, and we encountered a few adults later, paddling around a bigger pond. Following a somewhat crowded footpath, we stopped to dig into some activity around a gently sloping hillside. In this area we found a few calling Bewick’s Wrens, a Beryline Hummingbird, Broad-billed Hummingbird, some lovely breeding plumage Lesser Goldfinches, and a singing Black-headed Grosbeak. So far, along with the common and garrulous Great-tailed Grackles, this list of birds could easily have come from Southeast Arizona, which is a region I have birded extensively. Beryline Hummingbird was really the only semi-rare hummingbird I missed while I was in Arizona however, so I was quite happy to pick them up here. I also was never lucky enough to find a vagrant Rufous-backed Robin in the state, so that was another much-appreciated lifer. 

Suddenly, we ended up in a busy market area. Vendors surrounded us, and speaker-blasted music drowned out the birdsong. Anxiously seeking shelter at a nearby creek, we were rewarded with a pair of calling Harris’s Hawks perched high up in some trees and being mobbed by grackles. Chapultapec is large enough to be separated by a highway into two distinct sections, and at this point I realized we were still barely into the eastern “Primera Seccion” of the park and would be potentially an hour’s walk from the better birding of the more interior “Tercero Seccion.” Greedily, and conscious of the clock ticking towards midday, we hired an Uber to take us into the western depths of Tercero.

                                                          This Harris's Hawk was calling from its high perch.

Upon arrival, within a small group of House Finches, Nada pointed out a diminutive Northern Beardless Tyrannulet (despite only getting into birding a few months ago, she’s really a natural). We found a little trail through a dense and hilly area, but we were approaching peak midday heat, and Nada’s prioritization of fashion over comfort brought her to the park in a stylish pair of cowboy boots, which while attractive, were actively digging a hole in her heel skin, and we both felt we ought to return to our Airbnb soon for recuperation. Bird activity had dropped off significantly, but before we left, we were able to get some great views of a decent sized group of Vermilion Flycatchers, including a stunning red male, along with some Vaux’s Swifts soaring high up in the sky amongst some of the more abundant Barn Swallows. While our lazy late morning visit did not produce the most impressive bird list, and despite a few sore misses such as Blue Mockingbird, under the circumstances we left the park happy with our efforts and comforted by a few lifers.

Our first afternoon in the city was less about birds than relaxation. We took some rest at our Airbnb before enjoying a local pizza dinner, and spent the remainder of the evening with Nada’s parents, who have no interest whatsoever in birding. We went to sleep giddy with anticipation for our following morning in the mountains.

Day Two:

We rose early on Saturday and took a thirty-minute uber from the valley within which Mexico City is nestled, up into the mountains to the entrance of Parque Nacional Desierto de los Leones, which was the site I was most excited to explore on our trip. Leaving the Uber, we were greeted by cool air and large montane conifers that triggered memories of the Pacific Northwest. On the outskirts of the park, we scanned the edge of the forest and found several lifer White-throated Thrushes, as well as a more familiar Hutton’s Vireo and Mexican Chickadee. Finding some American Robins felt like bumping into an old friend at a party full of people you don’t know.

A muddy dirt trail took us further into the vast forest. Western Flycatchers were calling all around, and the first bird we laid eyes on in the area was the much anticipated and stunning Crescent-chested Warbler. After enjoying this unique and colorful species, its distant cousin, the Golden-browed Warbler, flew in and gave phenomenal views as well. Later, a skulky Central Mexican endemic Russet Nightingale-Thrush gave brief views as it hopped through a dense bush. Nada unfortunately missed this individual, but we heard several others singing throughout our hike and eventually got good views of a couple.

Taking pride in being a snobbish birding purist, I don’t tend to use the Merlin app much to identify bird calls, but Nada was using it throughout our walk and I must say it helped us quite a bit, as I hadn’t done my homework and was largely unfamiliar with these unique Mexican bird sounds. At one point Merlin recognized the high-pitched whistle of a Brown-backed Solitaire, but it was far too high up in the canopy for us to pick out. As we strained for a window through the tree branches to see where it was perched, another individual called from deeper in the forest. After a brief duet, we could hear our closer bird’s calls growing more distant, as it presumably flew off towards the other solitaire to sort out their territorial dispute. We heard one other solitaire later in our walk but sadly left our trip without a view of any.

Birding the forest was quite difficult, as the healthy towering conifers did a good job of obscuring many of the little flitting birds in the canopy. Our main target of the day (and really of the entire trip) was the stunning endemic Red Warbler, and for the first hour or so of our walk, we had merely been teased by its delicate song and had yet to get a view of one. I knew from preparing for this trip that these warblers could be observed quite reliably at this site, but I had the same impression about the solitaires that escaped me, and as time passed, I couldn’t help wondering, would we really leave without laying eyes on one? Suddenly, we noticed a darting flash of bright red through the branches. This was it; I would have to act quickly. Scrambling to get an angle, the bird obliged, and we were blessed with incredible views for a solid few minutes before the little gem took off. Red Warblers are a unique highland specialist with a small range, disjunct between a Sinaloa population to our north and a somewhat contiguous central/southern population that extends from Guadalajara in the west to Oaxaca in the east. The northern subspecies has a dark gray to black cheek, while our southern subspecies’ cheek is a handsome silvery-white. Needless to say, I was buzzing, and if I did not lay eyes on a single other bird for the rest of the trip, I would have returned home satisfied.

                    I debated whether I should include this horrible record shot, but this warbler is simply too cool not to share!

Luckily that would not be an issue, as the last few hours of our walk produced several more species, notably two endemics that were significant targets for us: the Green-striped and Rufous-capped Brushfinch. These charismatic sparrow-relatives are always a treat to see, and their phylogeny is quite interesting, but maybe I’ll save that topic for another blog post.

I felt as though I could’ve birded there all day, but around noon we decided we ought to call it, and we returned to the city to meet Nada’s parents for lunch. While birding is conventionally viewed as a hobby, it can also be viewed as an affliction, or a derangement. In hard cases such as myself, the birding sickness tends to manifest in especially devious ways during trips abroad. As I sat with Nada and her parents at lunch discussing plans for the rest of the day, I was at war internally. Her parents suggested making a trip to the Frida Kahlo museum, or perhaps some ancient ruins outside the city, and the human part of me thought that sounded nice, but the birder in me rebelled. I tried to quiet him, to force him back into the depths of my being, but he was too powerful. “You know, the university botanical gardens are quite close to the Frida Kahlo museum, perhaps we could go for a short walk there first?” I blurted out. “Oh, ok. What can we do there?” was Nada’s mom’s polite challenge. “Well, they have all these lovely plants, and the university campus is famous for its unique architecture,” was the angle I came up with. “Hmm. Ok, why not.” Lukewarm at best, but the drug addict isn’t concerned with debasing himself as he hunts for a fix, and neither was I at that point.

The four of us crammed into an Uber and off we went. On the way there I realized with frustration that the gardens close at 3 pm on Saturdays, and we would be arriving just after 2. I would not have nearly as much time as I would have liked. When we arrived, before we even passed through the gate, I noticed some kind of flycatcher perched on a tree branch. Upon closer inspection I was glad to see it was a lifer Buff-breasted Flycatcher. Staying true to the theme of picking up birds I had missed while living in Arizona, Buff-breasted Flycatchers had been a sore subject for me ever since my brother and I missed them on our trip to the Chiricahua mountains a few years ago. I was glad to finally see one here.

The botanical gardens ended up being quite pleasant. The first area we explored was a nicely constructed desert landscape, featuring many beautiful desert plants and some Curve-billed Thrashers, which were lifers for Nada and a new bird for our trip list. It was here that we found our first herp of the trip, the oddly named crevice swift (Sceloporus torquatus). These are large spiny lizards endemic to central Mexico, and even Nada’s parents seemed excited to see them basking gingerly on the black volcanic rocks. With less than an hour to explore the huge gardens, Nada and I agreed that we ought to break from her parent’s leisurely pace. As we sped off deeper down the main path, we quickly came upon a group of one of my top remaining targets for the trip, the Gray Silky-flycatcher. These unique and elegant long-tailed birds flew into the crown of a large tree near us, and despite flying actively in a chaotic jumble, Nada and I both eventually got great views of them before they left to find more berries elsewhere.

Stopping in a little cul-de-sac with some flowers that looked good for hummingbirds, we found a Beryline and a Broad-billed Hummingbird. In the distance I noticed a Ladder-backed Woodpecker climbing up a bare tree branch. A pair of Canyon Towhees hopped around on the ground nearby, foraging seeds seemingly without a care in the world. Before I knew it, it was nearly 3 pm, and reluctantly, we had to turn around. On our way back to the entrance, we passed another flowering bush, and noticed a few people gathered around pointing to something in it. As we approached, we found a lovely female White-eared Hummingbird buzzing from flower to flower, as well as a Cinnamon-bellied Flowerpecker delicately hopping amongst the branches. It was nice to see two distinct strategies for consuming nectar side-by-side. I left the gardens wishing for more time but supremely happy with the birds (and one lizard) we managed to encounter.

                                      Cinnamon-bellied Flowerpecker. Low bar but probably my best picture from the trip.

These early afternoon periods in the day seemed to be when the altitude affected us flatlanders most, so when we got to the Frida Kahlo museum a little later and were told they were fully booked, the prospect of a little afternoon rest more than made up for our disappointment. Sunday would be my 27th birthday, but since we would be traveling for most of it, we decided to enjoy a cheeky early birthday dinner on Saturday night. Nada and I went to El Califa de Leon, a famous and bustling taqueria known among other things for their wide selection of vegan options. Naturally, we went a little overboard and sampled quite a few of them. They were all delicious, but what blew us away most was the spread of six or seven different vibrant salsas that accompanied the meal. We fell asleep that night doing our best to avoid the harsh truth that the following morning would be our last for the foreseeable future in CDMX, but with a resolve to make the best of our final bit of birding.

Day 3:

Our last birding session of the trip would be along the Canal Nacional, in the southeast part of town. This stop was mainly driven by an obsession I had developed over the last few weeks with finding a Sparkle-tailed Hummingbird. This stunner is not so easy to see anywhere in Mexico City, but it seemed like our best bet would be to scan the flowering plants along the lush footpath accompanying the canal. It was a beautiful morning, but unfortunately, we were not the only ones with the good sense to spend it there. Maybe the park is less crowded on weekdays, but on that Sunday morning it was full of people strolling and jogging. Not quite ideal for birding, but it was still enjoyable, and we quickly picked up a few common but new species for our trip list, including Green Heron, Common Gallinule, Song Sparrow, and Eurasian Collared Dove. It was also funny to see a large group of exotic Monk Parakeets noisily foraging in some trees.

Hearing a song that I did not recognize, I looked up and was ecstatic to find a last-minute lifer Black-backed Oriole, with its striking orange belly and black upperparts. Sadly, it quickly flew off, and we did not see another. The canal was lined with orange flowering plants, and I could not help being repeatedly annoyed by the lack of hummingbirds in them. Any bit of motion around these flowers, whether from insect or wind or Song Sparrow, drew my anxious attention. Finally, I noticed the unmistakable emerald back of a hovering hummingbird and wished with all my might for a sparkle-tailed but discovered it was merely a common Beryline. As birders we must be vigilant against jadedness, and I had to remind myself that Beryline Hummingbirds are extremely cool too, despite my undeniable disappointment. As we left the park for the airport, even without my hummingbird target, it was hard to feel anything other than satisfaction overall.

                                           Beryline Hummingbirds were the most common hummingbird in the area.

My first visit to Mexico was tantalizingly brief but very lovely. The 58 species we recorded over three days were a nice introduction but only added fuel to my fire for birding more extensively in the country. Maybe Oaxaca ought to be next?

Here’s my eBird trip report from the weekend:

https://ebird.org/tripreport/544132